The following points highlight the five important theories about heredity genetics. The theories are: 1. Preformation 2. Epigenesis 3. Acquired Characters 4. Pangenes 5. Germplasm.
Heredity Genetic: Theory # 1. Preformation:
This theory was proposed by two Dutch biologists, Swammerdam and Bonnet (1720-1793). This theory states that a miniature human called homunculus was already present in the egg and sperm. In other words, a miniature human was performed in the gametes.
The development of zygote resulted only in the growth of miniature human, who was already present in the egg and sperm. However, this theory was soon given up because this could not be proved scientifically.
Heredity Genetic: Theory # 2. Epigenesis:
This theory was advocated by Wolff (1738—1794), a German biologist. This theory states that egg or sperm cells do not contain miniature human. In other words, egg or sperm cells are undifferentiated.
The differentiation into various organs or parts takes place only after fertilization from the zygote resulting into development of adult tissues and organs. This concept is known as epigenesis which is universally accepted.
Heredity Genetic: Theory # 3. Acquired Characters:
This concept was proposed by Lamarck (1744—1829), a French biologist. This theory states that a new character once acquired by an individual shall pass on to its progeny. It means if a man develops a strong muscle by exercise all his children will have strong muscle.
On the other hand, if a person becomes weak all his children will be weak. This theory was disproved by Weismann. He cut the tail of mice for 22 successive generations and always got the baby mice with tail. Thus, this theory was soon given up.
Heredity Genetic: Theory # 4. Pangenes:
This theory was proposed by Charles Darwin (1809—1882), an English naturalist. This theory states that very small, exact but invisible copies of each body organ and component (called gemmules) are transported by the blood stream to the sex organs. These invisible copies of each body organ are called the gemmules.
These gemmules are assembled in the gametes. After fertilization these gemules move out to different parts of the body resulting in the development of respective organ. A defective gemmule will lead to the development of defective organ in an individual. This theory was given up because it did not have scientific basis.
Heredity Genetic: Theory # 5. Germplasm:
This theory was advocated by August Weismann (1889), a German biologist. This theory states that body tissues are of two types, viz., germplasm and somatoplasm. The germplasm refers to the reproductive tissues or cells which produce gametes.
The somatoplasm includes all other body tissues which are not related to sexual reproduction. Thus, transmission of characters from one generation to other takes place only through germplasm. Any change in the germplasm will lead to change in the next generation. This theory is accepted in a broad sense.